The Jaffe reaction (creatinine and alkaline picrate) was studied to elucidate its spectral mechanism and to determine which functional groups on cefoxitin and cephalothin react with picrate. Suitable evidence was obtained to infer that of the carbonyl groups on cefoxitin or cephalothin, the lactam ring is responsible for the flow reaction and the acetamido group is responsible for the fast reaction. An autocatalytic reaction between picrate and carbohydrates was identified and studied.